It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from wear, soot deposits and acid build up. Often, such surface active additives including zinc dialkyldithiophosphates can have harmful effects on bearing corrosion or friction performance. As friction increases, fuel economy tends to decrease. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). The addition of known friction modifiers (such as glycerol monooleate), are believed to lower the coefficient of friction. However, friction modifiers may have deleterious effects as well such as competing with the antiwear agent or bearing corrosion (typically containing lead and copper).
Various attempts have been made to reduce corrosion caused by ashless additives. These attempts include those disclosed in US Patent Application US 2004/038835; U.S. Pat. Nos. 3,966,623, 3,896,050, U.S. Pat. No. 4,012,408; and European publication EP 1 642 954.
US Patent Application US 2004/038835 discloses certain 1,2,4-triazole metal deactivators are especially non-aggressive towards lead engine parts such as bearings. The inclusion of certain 1,2,4-triazole compounds allows the co-use of corrosive additives such as sulfur-containing additives and vegetable oil-derived friction modifiers.
U.S. Pat. No. 3,966,623 discloses improved copper corrosion properties by employing a lubricant that contains a combination of an alkenyl or alkyl primary amine derivative salt of 2-mercaptobenzothiazole, and 2,5-bis-hydrocarbyldithio-1,3,4-thiadiazole. This combination is suitable for reduction of copper corrosion caused by additives with detergent, dispersancy, load carrying and lubricity functions. These additives may be corrosive in themselves and/or break down during use into corrosive substances which result in severe corrosive attack.
EP 1 642 954 discloses a fluid composition comprising at least one hydroxy-substituted carboxylic acid. The hydroxy-substituted carboxylic acid provides at least one property chosen from rust inhibition, corrosion inhibition, improved lubricity, and improved lead compatibility.